no oil on rocker's drivers side only? 360 magnum

the engine was a running/used engine that he pulled and sold the truck it was in. I pulled the intake and made sure the galley plug was there. I got brand new lifters and the same thing. I put the lifters in with the oil holes facing down(towards the center correct?) When I got the engine I was told that the heads were la's converted to magnum's but after looking at them, they are clearly magnum heads on a magnum block. That being said I know I installed the la head gaskets. Could there be a oil passage that is being blocked by the headgasket. thanks for the help guys.